In the past, a radar apparatus provided in a vehicle derives moving targets corresponding to other vehicles (hereinafter referred to as a preceding vehicle) traveling in the same traffic lane as a traveling subject vehicle ahead of the subject vehicle, and stationary targets (hereinafter referred to as a stationary target) corresponding to other vehicles (hereinafter referred to as a stopped vehicle) stopped in the same traffic lane as the traveling subject vehicle ahead of the vehicle. Further, the radar apparatus derives stationary targets (hereinafter referred to as an on-road object) corresponding to objects (hereinafter referred to as an on-road object) installed on a road surface of the traffic lane, such as a manhole or a joint member for jointing a road and a road, ahead of the subject vehicle traveling in the same traffic lane.
In a case where the stationary target is derived from the moving target and the stationary target, the radar apparatus derives target information including position information, such as a distance of these targets, and relative speed information to output the vehicle information to a vehicle control device capable of controlling a behavior of the vehicle. The reason why the radar apparatus outputs the target information about the moving target or the stationary target to the vehicle control device is that the vehicle control device executes, for example, the control of ACC (Adaptive Cruise Control) or PCS (Pre-Crash Safety System) based on the target information. That is, the radar apparatus outputs the target information about the moving target to the vehicle control device to execute the control of the ACC follow-up traveling in a state in which inter-vehicular distance between the vehicle and the preceding vehicle is constantly maintained. In addition, the radar apparatus outputs the target information about the moving target to the vehicle control device to execute the control of the PCS capable of preventing collision between the stationary target and the vehicle or alleviate the effect of the collision.
Since the vehicle cannot cross over the on-road target, contrary to the stationary target, the on-road target is one which is not necessary to be output to the vehicle control device from the radar apparatus. That is, in the case where the vehicle control device executes the control of the ACC or the PCS, the on-road target is the stationary target other than the control subject.
For this reason, to correctly determine whether the derived stationary target is the stationary target or the on-road target, the radar apparatus judges a kind of the target, for example, based on a signal level relating to a receiving signal of the stationary target and a signal level relating to a receiving signal of the on-road target.
Specifically, since the stationary target has a height of a vehicular body to a certain extent, a reception antenna of the radar apparatus receives a reflection wave (hereinafter referred to as a direct wave) outputted from the transmission antenna and then directly reflected from the stationary target, or receives a reflection wave of multi-path (hereinafter referred as a multi-path wave) reflected from the road surface at which the reflection wave from the stationary wave is reflected.
Since the multi-path wave is received by the reception antenna in delayed timing relative to the direct wave, a frequency and phase of the multi-path wave are different from those of the direct wave. In a case where the receiving signal (hereinafter referred to as a direct signal) of the direct wave and a receiving signal (hereinafter referred to as a multi-path signal) of the multi-path signal which have the frequency difference and the phase difference are synthesized, a receiving signal having a given amplification variation is generated. As the vehicle approaches the stationary target and thus a distance between the vehicle and the stationary target becomes short, the signal levels of the direct signal and the signal level of the multi-path signal are increased. For this reason, the amplification variation of the receiving signal of the stationary target is characterized by being increased as the distance between the vehicle and the stationary target becomes short.
Meanwhile, since the on-road target hardly has any height in a height direction of the vehicle, the radar apparatus does not receive the multi-path wave, but receives the direct wave. for this reason, the receiving signal of the on-road target becomes a value of a substantially constant signal level with almost no amplitude variation. Further, as the distance between the vehicle and the on-road target becomes short, the value of the signal level of the receiving signal is increased, but in a case where the distance between the vehicle and the on-road target is below a predetermined distance, the on-road target has hardly a height relative to the height of the vehicular body, so that the on-road target is out of a transmission region of the transmission wave of the radar apparatus. As a result, the value of the signal level of the receiving signal is characterized by being abruptly decreased if the distance of the on-road target is below the predetermined distance. That is, the receiving signal of the on-road target is characterized in that the amplitude variation is hardly generated, and if the distance between the vehicle and the on-road target is below the predetermined distance, the value of the signal level is abruptly decreased. A technology for interrupting the control of the vehicle to the on-road target is disclosed in the case where the stationary target is judged as the on-road target based on the above-described features of the receiving signal levels of the stationary target and the on-road target (e.g., see Patent Document 1)
Patent Document 1: Japanese Patent Application Publication No. 2001-191876A
However, in a case where a slope of the road surface on which the vehicle exists is different from that of the road surface on which the on-road target exists, the feature of the receiving signal of the on-road target may be changed. Specifically, the road surface on which the on-road target exists has a slope such as an ascent, contrary to the road surface on which the vehicle exists, since the on-road target exists at a high position to a certain extent in a height direction (hereinafter referred to as a reference direction) on the basis of the road surface on which the vehicle exists (i.e., flat road surface with respect to the road surface of the ascent on which the on-road target exists), the on-road target has a certain height with respect to the position on which the vehicle exists. In the case where the on-road target exists on the road surface of the ascent, the direct wave and the multi-path wave are generated from the transmission wave of the on-road target, similar to the stationary wave, and are synthesized to generate a receiving signal having an amplitude variation.
As the vehicle comes close to the on-road target and thus the distance between the vehicle and the on-road target becomes short, the signal levels of the direct signal and the multi-path signal are increased. Therefore, there is a feature in that as the distance becomes short, the amplitude variation of the receiving signal resulted by synthesizing the direct signal and the multi-path signal is relatively increased. That is, the feature of the signal level relating to the receiving signal of the on-road signal existing on the road surface having the slope of the ascent is identical to the feature of the signal level relating to the signal level relating to the receiving signal of the stationary target. For this reason, the radar apparatus may judge the receiving signal as the stationary target other than the control subject of the vehicle control device, and judges the target information about the stationary target which is not output to the vehicle control device, as the stationary target of the control subject to output it to the vehicle control device.